The present invention relates to bistable cholesteric liquid crystal displays and their electrical drive schemes.
Currently, information can be displayed using assembled sheets of paper carrying permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information, however magnetically written data is not visible.
Flat panel displays use two transparent glass plates as substrates. In a typical embodiment, such as one set forth in U.S. Pat. No. 5,503,952, a set of electrical traces is sputtered in pattern of parallel lines that form a first set of conductive traces. A second substrate is similarly coated with a set of traces having a transparent conductive coating. Coatings are applied and the surfaces rubbed to orient liquid crystals. The two substrates are spaced apart and the space between the two substrates is filled with a liquid crystal material. Pairs of conductors from either set are selected and energized to alter the optical transmission properties of the liquid crystal material. Such displays are expensive, and currently are limited to applications having long lifetimes.
Fabrication of flexible, electronically written display sheets using conventional nematic liquid crystals materials is disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent indium-tin-oxide (ITO) conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be thin glass, but in practice have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential is applied to opposing conductive areas to operate on the liquid crystal material and expose display areas. The display uses nematic liquid crystal materials, which ceases to present an image when de-energized. Privacy windows are created from such materials using the scattering properties of conventional nematic liquid crystals. Nematic liquid crystals require continuous electrical drive to remain transparent.
U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a chiral nematic liquid crystal in polymeric domains contained by conventional patterned glass substrates. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal conic state. Chiral nematic material has the capacity of maintaining one of the given states in the absence of an electric field.
In xe2x80x9cLiquid Crystal Dispersionsxe2x80x9d, World Science, Singapore, 1995, page 408, Paul Drzaic discusses the electrical drive of cholesteric liquid crystal displays. Drzaic also states on page 29 that xe2x80x9cThe use of gelatin, however, creates a material that is too conductive for practical use in electrically addressed PDLC systems.xe2x80x9d. Drzaic further states xe2x80x9c. . . actual displays require AC signals to prevent electrochemical degradation.xe2x80x9d Subsequent patents follow Drzaic""s assumptions. Later patents such as U.S. Pat. Nos. 5,251,048 and 5,644,330 and 5,748,277 all require AC fields having a net zero unipolar field for matrix cholesteric liquid crystal displays to prevent ionic destruction of the display. The cited patents have display structures formed using expensive display structures and processes applicable to long-life situations which require AC drive schemes.
The drive schemes require that each element be written using alternating electrical fields that provide a net zero field across the display to prevent ionic migration. AC drives require large numbers of power supplies and large numbers of switching elements per line.
It is well known that the cholesteric liquid crystal displays have two optically bistable states. Multiple color can be obtained by color pixelization [Chien et al., SID 95 Digest, XXVI, pp.169-171 (1995)] or triple stack [Huang et al., ASIA DISPLAY 98, pp.883-886 (1998)]. Full colors of cholesteric displays rely on gray scales of individual color. Therefore, the development of a display with gray scale created using simple drive waveforms benefits from a lower cost electrical drive circuitry. Gray scale has been achieved with a binary dithering method [Z. Yaniv et al. Proc. of International Display Research Conference, 113 (1995)]. However, this method significantly sacrificed the effective resolution. Gray scale has also been achieved by controlling the proper mixing ratio of the planar (reflective) state and the focal conic (non-reflective) state within one pixel. But with disclosed cholesteric liquid crystal displays, the response of the reflectance to the driving voltage is dependent on the initial state. Yang et al. stated in Appl. Phys. Lett. 64, p.1905 (1994) that xe2x80x9cFor the gray scale display, first the display has to be freshened, that is, a high voltage pulse applied to all the pixels to drive them into the reflecting state.xe2x80x9d In order to achieve a gray scale, a driving voltage is required to switch the display into an intermediate state. For example, cholesteric displays were first switched into either the planar state or the focal conic state by a proper voltage pulse before a selection phase which selects the gray scale was applied [Gandhi et al., Asia Display 98, pp.127-130 (1998)]. In another example, the cholesteric displays were first switched into the homeotropic state by a sufficiently high driving voltage [Huang et al., SID Digest 98, pp.810-813 (1998)]. This high voltage functioned as erasing memory.
Commonly assigned U.S. patent application Ser. No. 09/723,389 filed Nov. 28, 2000 by Johnson and Stephenson, the disclosure of which is incorporated herein by reference, disclosed that the planar state and the focal conic state could be achieved by a DC pulse.
It is an object of the present invention to provide low cost memory displays generated using coated polymer dispersed cholesteric liquid crystals on flexible substrates, and the response of the reflectance of the said displays to the driving voltage is independent of the initial state in a certain range of driving voltage.
It is another object of the present invention to provide a simpler, lower cost method of achieving gray scale images, which is the basic requirement for a color display.
It is another object of the present invention to provide a simpler, lower cost method of achieving full color images using the above mentioned gray scale displays with different colors.
These objects are achieved by a cholesteric liquid crystal display for producing gray scale images, comprising:
a) a layer including a polymeric host material, cholesteric liquid crystals in the host material and having a selected domain size, and a surfactant, the materials in the layer selected to cause the cholesteric liquid crystals to be effective in a number of different states of reflectivity and will remain in any given state until a field is applied;
b) electrodes disposed relative to the layer for applying an electric field to the layer when a voltage is applied to the electrodes; and
c) means for applying at least one voltage pulse to the electrodes which cause the direct change of the cholesteric liquid crystals from any initial state to a particular state within a selected gray scale.
The present invention discloses a display for producing gray scale images by a DC pulse without need to erase memory (without need to first switch liquid crystals into an intermediate state).
Gray scale images of the said displays can be achieved by a simple combination of on-voltage pulse and off-voltage pulse without any voltage required before this selection-voltage.
The invention reduces the number of voltages required to drive such a display as well as reducing the number of voltage switching elements.
It is a feature of the present invention that it eliminates the need to work with glass substrates with associated complex manufacturing techniques. The present invention by making use of flexible polymeric substrates permits the use of unipolar fields to change reflectance. The present invention facilitates the life of displays as well as reducing the associated cost of the drive circuit.